Three-phase motors have been used for many years, but primarily in industrial sizes. Electricity in the home was first used for production of light and single-phase AC was the commercial power supplied to homes. As a result, now that AC power is the prevailing power supplied to homes, it is single-phase electric motors which have been used to power the various appliances which have become commonplace. Therefore, small three-phase motors of the fractional or subfractional horsepower sizes have received very little development. The three-phase motors are therefore generally larger sizes which are complicated in construction, and to merely make a small size AC motor of this type would make an extremely expensive small motor which would be uneconomical, and hence generally unmarketable.
The small size, single-phase AC motors are often shaded pole induction motors which are relatively inefficient. Subfractional horsepower capacitor induction motors are more efficient, but even these may be quite low in efficiency if not balanced at the operating speed. Since the start winding current is unstable in both magnitude and phase angle, as shown in my 1929 AIEE Paper, it can be balanced at only one speed at a condition where the main and start winding ampere turns are equal and 90 degrees apart in time, or close to it. At its best, the capacitor motor can approximate to the efficiency of a symmetrical polyphase motor at one speed but at all other speeds it will be less efficient because a polyphase motor is balanced at all speeds. When a capacitor motor is unbalanced, its torque contains a 120 Hertz component whose magnitude is given by my paper, and unless the motor is precisely elastically mounted, rotationally it will cause its supporting surface to radiate a loud 120 Hertz hum. The torque of a polyphase motor has no 120 Hertz component, and thus no hum. In many applications, this 120 Hertz hum is disturbing. Additionally, subfractional horsepower, single-phase motors often have a third harmonic dip in the speed-torque curve which is difficult to remove,and hence the motors are often unsatisfactory for driving loads which have a high starting torque requirement.
A three-phase motor never has a third harmonic or any multiple thereof. Therefore, it never has a third harmonic dip.
Subfractional horsepower motors in the past have been relatively inefficient because there was no reason in the marketplace to require an efficient motor, since electrical power was quite inexpensive. Now it is realized that electrical power is not exhaustible and more efficient electrical appliances are being demanded. The prior art sizes often relatively few turns of relatively large wire, and hence were quite inefficient, but this was the least expensive way to manufacture such motors. A number of prior art single-phase motors had the disadvantage of requiring the expense of a capacitor and its mounting, or requiring the expense of a starting winding and a speed-responsive switch to disconnect the starting winding after the motor had reached full speed. Such motors required space for the starting winding which was not used during normal running. The capacitor was often mounted to the outside of the motor.
U.S. Pat. No. 2,486,435 has suggested a construction for a three-phase motor, either with salient poles or with a distributed winding, and because of the distributed winding structure illustrated, this was presumably for larger sizes of AC motors, such as integral horsepower motors. Such prior art construction was concerned with an attempt to provide a variable speed by shunting some of the flux away from the rotor, and proposed a construction which was impractical because it did not have a uniform value flux as it rotated around the stator during one cycle of the input power.